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Ionization

Ionization. Measuring Ions. A beam of charged particles will ionize gas. Particle energy E Chamber area A An applied field will cause ions and electrons to separate and move to charged plates. Applied voltage V Measured current I. I. +. E. A. -. V.

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Ionization

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  1. Ionization

  2. Measuring Ions • A beam of charged particles will ionize gas. • Particle energy E • Chamber area A • An applied field will cause ions and electrons to separate and move to charged plates. • Applied voltage V • Measured current I I + E A - V

  3. Ion – electron pairs created will recombine to form neutral atoms. High field needed to collect all pairs V > V0 Uniform particle beam creates constant current. Saturation current I0 Saturation I I0 V V0 Ion recombination Saturation

  4. A uniform beam is defined by fluence rate and energy. Intensity is the product The number of ions depends on the gas. Ionization energy W The saturation current is proportional to intensity Energy per area per time: The number of ion pairs N is: The saturation current is: Saturation Current

  5. W values measure the average energy expended per ion pair. Electrons uniform with energy Protons above 10 keV similar to electrons W for heavy ions increases at low energy. Excitation instead of ionization Gas WaWb (eV/ion pair) He 43 42 H2 36 36 O2 33 31 CO2 36 33 CH4 29 27 C2H4 28 26 Air 36 34 Ionization Energy

  6. Typical Problem A good electrometer can measure a current of 10-16 A. What is the corresponding rate of energy absorption in a parallel-plate ionization chamber with W = 30 eV/ip? Answer The energy rate is related to the intensity. (10-16 C/s)(30 eV)/(1.6 x 10-19 C) = 1.88 x 104 eV/s Equivalent to one 18.8 keV particle per second Electrometer

  7. Many household smoke detectors are ionization chambers. Electric field from a battery 241Am alpha source (.5 mg) Smoke interrupts saturation current through recombination. Smoke Detector howthingswork.com

  8. Liquid noble gases can be used in ionization chambers. Liquid argon, krypton, xenon An applied field of 1.1 MV/m used to suppress scintillation in liquid Ar. Focus on an example from the Dzero electromagnetic calorimeter. Liquid argon parameters Density 1.41 g/cm3 Boiling point 87 K W value 23.6 ev/ion pair Liquid Argon

  9. Uranium plates are alternated with readout pads. Separated by liquid argon Readout pads are 5-layer printed circuit boards. Outer readout pads Inner layer readout wires Ground planes to reduce crosstalk Resistive coat at 2.5 kV Uranium Cell 4.0 mm 2.3 mm 4.3 mm depleted uranium readout pad liquid Ar gaps

  10. Uranium acts as an absorber. Density 19.05 g/cm3 Interaction primarily in uranium 4 cm for electromagnetic Shower particles ionize liquid argon in the gaps. Measured on circuit board pads Shower Production incident particle depleted uranium

  11. The energy loss in the uranium is much greater than in the argon. Ionization is a sample of the particles in the shower. Readout signal is proportional to a sample of the shower energy. A sampling calorimeter loses some resolution due to statistics. Gains in flexibility to construct optimal layer thicknesses Sampling Calorimeter

  12. High Voltage Response

  13. - + Pulse Mode • Ionization signals are read out as individual pulses. • Cell is a capacitor CD • Total charge dQ proportional to ionization • Charge sensitive preamp integrates current pulse to get charge. • Measure as voltage change CF iin vout CD

  14. Integrating the charge means selecting a sample time and initial voltage. 2.4 ms Subtract the baseline voltage The distribution with no input signal is the pedestal. Asymmetric due to uranium noise Pedestal

  15. The pedestal is not constant. Variation of the pedestal contributes to statistical error. Depends on cell capacitance High voltage on picks up uranium noise Cell Noise

  16. Total energy for a particle is due to a sum of N channels. Resolution varies with total energy Signal variance S2 depends on a number of sources of error. Statistical channel error s Channel crosstalk error c Resolution

  17. Electrons of known energy are used to calibrate the cells. Initial digital counts Aj Cell calibration bj Tower calibration a Offset for other material d Compare beam momentum to measured energy for various energies. Electromagnetic Calibration

  18. Beam Response

  19. Energy resolution is measured as a fraction s/E. From mean and standard deviation fit to Gaussian Resolution is fit to a quadratic as a function of momentum Channel-to-channel variation C Statistical sampling S Energy-independent noise N Fit results: C = 0.003 ± 0.003 S = 0.157 ± 0.006 N = 0.29 ± 0.03 Quoted resolution: Measured Resolution

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